Industrial process and apparatus



M. E. FUENTEVILLA ET AL 3,415,445

INDUSTRIAL PROCESS AND APPARATUS Dec. 10, 1968 Filed. Nov. 21, 1966 IOO INVENTORS. MANUEL E. FUENTEVILLA ANTHONY J. SARACENO PUM INVENTION SUCTION PRESSURE, MM Hg. ABSOLUTE PRIOR ART ATTORNEY.

United States Patent 3,415,445 INDUSTRIAL PROCESS AND APPARATUS Manuel E. Fuentevilla, Cherry Hill, N.J., and Anthony J. Saraceno, Devon, Pa., assignors to Pennsalt Chemicals Corporation, Philadelphia, Pa., a corporation of Pennsylvania Filed No. 21, 1966, Ser. No. 595,961 11 Claims. (Cl. 230-207) ABSTRACT OF THE DISCLOSURE A vacuum pump system for operation at temperatures above 212 P. which is devoid of any expedient to avoid condensation or to revaporize water or solvent vapor (such as water jackets or gas ballasting) whereby a lubricant maintains the seal between the pump rotor and the pump chamber wall.

This invention involves operating a mechanical vacuum pump at relatively high temperatures. More specifically this invention relates to operation of a mechanical vacuum pump at temperatures above 212 F. and lubricating the pump with a lubricant comprising a silicone and a thickening amount of a realtively low molecular weight chromium phosphinate copolymer.

It is surprising that a silicone based fluid can be used to advantage in a vacuum pump system because heretofore silicone fluids have been too limited in metal to metal lubricity capabilities for continuous vacuum pump operation.

In the past it has been customary to operate vacuum pumps of the type, for instance, shown in the US. Patent 3,156,410, at temperatures from 140 to 180 F. Such a pump is shown in FIGURE 1 of the present application. The pump has included special cooling chambers A through which circulating water flows to withdraw heat from the pump and keep it down to the indicated temperature. The reason for operating at this low temperature has been that available lubricants such as petroleum oils with or without additives have not been able to withstand temperatures at which such a pump would normally operate.

The conventional vacuum pump oil is a low vapor pressure hydrocarbon lubricant with rust and emulsionresistant additives. Such an oil is limited to operation under approximately 180 F. The limit to the upper temperature is necessary to prevent formation of a varnish deposit in the pump and to maintain proper viscosity in order to effect a vacuum seal between the pump rotor B and pump chamber walls C (FIG. 1). Furthermore, in the operation of conventional vacuum pumps where water cooling is required, pump efliciency is often adversely affected and pump clearances are limited to higher than desired values due to thermal gradients within the system.

By operating the pump at low temperatures in order to accommodate to the characteristics of the lubricant, certain difliculties arise. Vacuum pumps are frequently used to remove inert gases from process systems which contain also water and solvent vapors. Under these conditions equilibrium portions of water and/or solvents condense in the pump oil and unless removed cut down on the capacity and ultimate suction pressure of the vacuum pump. As the pump oil circulates from the pumping chamber to the oil reservoir which is at atmospheric pressure and returns to the pumping chamber, condensed solvents revaporize in the pumping chamber. These recycling vapors limit the pump capacity. In some cases the condensation of water and solvent is extreme and the lubricating qualities of the oil are diluted causing mechanical 3,415,445 Patented Dec. 10, 1968 failure of the close fitting parts in addition to performance limitations.

Several means have been employed to rid the vacuum pump oil of such contaminants. The most common technique, called gas ballasting, has involved the introduction of air to the discharge side of the moving piston as at D in FIGURE 1. The additional quantity of air causes an adiabatic temperature rise in the pumping cycle and tends to prevent condensation of water and solvents in the oil. The contaminants are thus carried out as equilibrium vapors. However, the use of gas ballasting reduces the capacity of the pump and limits the ability of the pump to handle large quantities of water and solvents where necessary.

Reservoir venting to remove vapors from the pump oil has been another technique used to combat the contaminant problem. However, such venting is limited in the amount of water that can be removed from the pump. Centrifuging as another alternative will remove water from the oil but the initial cost and maintenance have been uneconomic. Distillation of the contaminant from the oil will remove the water and solvent, but will require heat energy and additional complex vacuum distillation equipment. Selective coalescing membrane and adsorption processes sulfer from the same capacity limitation plus a tendency to foul because of impurity build-ups.

Thus because the long-sought-for pump lubricant has not been discovered, pump operators have satisfied themselves to limit the capacity of their pumps and operate in the to range using the various indicated techniques to avoid condensation of water and solvent vapors and to vaporize condensed water and solvent vapors in the oil reservoir D (FIG. 1) to the degree possible without affecting the oil.

In summary the search for a suitable vacuum pump system operating at high temperatures-that is above 212"- at which water vapor and contaminants would not condense into the pump lubricating oil has been a long and unsuccessful one. The incentive for such a search is clear: if the pump were operated at a high temperature, there could be increased pump capacity and lower vacuums. There could also be virtually wanton disregard of the type of vapors introduced to the inlet of the pump. Therefore the pump could be used on dirty processes, that is in areas heretofore restricted because of the condensation of vapors into the oil.

The search for such pump system has had for its objective a set of very exacting characteristics which include:

(a) Ability to operate at a very low vapor pressure at elevated temperature.

(b) Maintaining an effective seal between moving parts during operation at elevated temperature.

(0) Resistance of the pump lubricant to oxidation, thermal degradation, and attack by common chemicals.

(d) Water immiscibility of the pump lubricant, the most common contaminant.

(e) Use of a stable, non-corrosive lubricant to preclude damage and contamination from diffusion through the vacuum pump back to high vacuum process environments.

The present invention involves the operation of a mechanical vacuum pump system at high temperatures. It is predicated on the discovery of a type of lubricating fluid which admirably qualifies for vacuum pump oil. The use of this fluid enables the operation of the pump at such a high temperature that water vapor and solvent vapor do not condense into the oil. This means that there is no longer necessity for water jackets or for gas ballasting or other expedient to avoid condensation or to revaporize the water vapor or solvent vapor. This makes possible the simplified less-expensive pump structure such as is shown in FIGURE 2 in which the lubricant main- 3 tains the seal between the rotor and the pump chamber wall 12.

Further objects of the invention will be understood from reference to the following specification including the drawings which are only exemplary of the advance of the invention and wherein:

FIGURE 1 is a sectional view of a vacuum pump of the prior art;

FIGURE 2 is a sectional view of a corresponding pump operated in accordance with the invention and showing the simplified construction made possible by the invention; and

FIGURE 3 is a graph illustrating the improvement in water vapor pumping capacity of a pump operated in accordance with the invention as compared to the capacity of a pump operated in accordance with the prior art.

Briefly, the invention is a process for operating a mechanical vacuum pump at temperatures above 212 P. which includes lubricating the pump with a liquid lubricant comprising a silicone and a thickening amount of a copolymer having an intrinsic viscosity in chloroform below about 0.5 and a water molecule, characterized by having at least two different bridging groups wherein each bridging group is the anion of an acid R R P(O)OH, where R and R are selected from the group consisting of alkyl and aryl containing from 1 to 18 carbon atoms and with the proviso that at least one alkyl group, and said copolymer being terminated at its ends by the :anion of an aliphatic carboxylic acid containing from one to four carbon atoms. The invention also embodies a pump apparatus lubricated with such a lubricant.

The class of liquid lubricants applicable to the present invention is thoroughly described in US. patent application Ser. No. 584,115, filed Oct. 4, 1966, now Patent No. 3,331,775, by Anthony J. Saraceno, and assigned to the same assignee as this application.

The above-mentioned application reports that it has been found that improved silicone fluids and improved silicone greases can be prepared by incorporating in a silicone fluid a relatively low molecular weight chromium phosphinate copolymer. More specifically, the invention in that application comprises a silicone fluid containing an improving amount of a copolymer having an intrinsic viscosity below about 0.5 in CHCl consisting of a doubly bridged chromium atom coordinated with a hydroxyl group and a water molecule, characterized by having at least two different bridging groups wherein each bridging group is the anion of an acid R P(O)OH, where R is alkyl or aryl containing from 1 to 18 carbon atoms and with the proviso that at least one bridging group contain at least one alkyl group, said copolymer being terminated at its ends by the anion of an aliphatic carboxylic acid containing from one to four carbon atoms. Thus the copolymers defined above may be represented by the following formula:

where R and R are alkyl or aryl groups, at least one of said R groups being alkyl, R is the anion of a carboxylic acid containing from one to four carbon atoms (e.g., formate, acetate, propionate and buty-rate), and where it is understood that the repeating units are random in nature. The above terms alkyl and aryl describing the R groups are intended to include substituted alkyl and aryl such as haloalkyl and haloaryl, including perhalo such as perchloro and perfluoro substituents. It will also be understood that the ratio of n to m, which are not necessarily integers, may vary, although the sum of n and in will be two in order to satisfy the valence requirements of the chromium atom. The symbol at merely indicates the polymeric nature of the formula shown.

For purposes of the present invention it has been found that the preferred range by weight of the copolymer is from 0.1% to 3% to keep the lubricant in the form of a liquid which may be circulated to achieve the desired sealing effect in the vacuum pump.

In a specific embodiment tests were initiated for abbreviated life cycles on a Model 146 Microvac pump made by Stokes with a nominal 23 c.f.m. capacity. The pump was operated at maximum power consumption points for approximately 350 hours at temperatures ranging from 200 F. to 220 F. The parts were examined and no wear attributed to the lubricant or high temperature operation was observed. In addition, this pump was operated on a water vapor over its usual pumping range without a gas ballast for extended periods of time, at temperatures from 220 to 275 F. The pumps suffered no loss in capacity due to water condensation. In these tests the liquid lubricant comprised methyl-phenyl silicone (Dow Corning Silicone Fluid 550) and 0.77% of a copolymer having an intrinsic viscosity in chloroform of about 0.15 and consisting of a doubly bridged chromium atom coordinated with a hydroxyl group and a water molecule. In the particular lubricant throughout this experiment one of the bridging groups of the polymer Was a dioctyl phosphinic acid anion while the other was a phenyl methyl phosphinic acid anion. The polymer was terminated at its end with an acetate anion.

In another experiment using the same lubricant as in the first example a 100 millimeter mercury absolute steam boiler was connected to the vacuum pump. An electric heating tape was wrapped around the pump body with rheostat control. Oil temperature was measured through thermocouples in a potentiometer. The pump handled approximately 6 pounds per hour water vapor at 100 millimeter mercury suction pressure for 6 hours at 235 F. oil temperature. Steam vapor issued from the mist separator exhaust. When the boiler valve was closed the vapor stopped and the suction pressure blanked off at less than 100 microns immediately. Steam vapors started again from the exhaust when the boiler was opened to the Microvac From these examples it was determined that the poly metal phosphinate additive mixed with the Dow Corning Silicone Fluid 550 is a satisfactory high temperature mechanical vacuum pump oil. It was also determined that the use of this fluid allows operation of a mechanical vacuum pump over their full operating range on 100% steam vapor.

A further preferred form of lubricant found useful with the high temperature operation of a mechanical vacuum pump and falling under the generic description above is one in which the copolymer has an intrinsic viscosity in chloroform of about .05 to .35 and the bridging groups are the anions of diphenyl phosphinic acid and dioctyl phosphinic acid, respectively, the copolymer terminating at its end in an acetate anion.

Other preferred species of the inorganic copolymers are noted in the above identified application. Some of the preferred species are:

Silicones used in the invention may be methylphenyl silicones, chlorophenylmethyl silicones, and the like.

It will be understood that the fluid lubricant in the vacuum pump system may be circulated externally of the pump housing for the purpose of heating it to maintain a sufiiciently high temperature for evaporation of the water vapor and solvents that enter the pump chamber. This procedure is often required when the pump is used under high temperature conditions since pumping energy inputs below about mm. Hg absolute suction pressure are not normally sufiicient to raise the pump body, rotor, and lubricating oil above 212 F. and also to counteract the radiative and connective heat losses. It will be understood, of course, that the external heater may be replaced with a cooling device in the event cooling of the chamber should be desired. In an alternative embodiment the temperature of the oil may be controlled by use of thermostatted internal electric heaters.

To illustrate the efiiciency of the pump operated in accordance with the invention a so-called hot pump as opposed to a pump operated in accordance with the prior art, a so-cal'led cold pumpj reference is made to FIGURE 3. The cold pump, the characteristics of which are shown in the FIGURE 3 graph by dotted lines, was operated at a temperature of 160 F. and was a 23 cubic feet mechanical vacuum pump of the Microvac type. In a conventional test, readings were made of suction pressure and pounds per hour of water vapor. Referring to the graph, the cold pump reached an upper limit at roughly 1.6 pounds per hour at approximately 40 millimeters mercury because the gas ballast air which is supplied to the chamber at the optimum rate is completely saturated with water vapor and can remove no more water from the pump. As a lower limit, it was found that the cold pump was unable to obtain a suction pressure much lower than 500 to 700 microns because it was repumping water vapor contained in the pump oil. In the hot pump, of course, the vapor for the vacuum chamber is not condensed into the oil because it is always kept above its condensation temperatures.

The present invention is more than the adaptation of a new lubricant to an old use. The invention has involved exhaustive screening and experimentation and has resulted in a completely new mode of operation of a conventional mechanical vacuum pump. This new mode of operation, namely operating at high temperatures, greatly simplifies the structure as shown in the figures and increases the efliciency of the pump because there is no necessity for conventional gas ballasting. Then, too, it makes feasible new uses of the pump on dirty processes, that is processes in which the vapors are loaded with contaminants which might otherwise condense in a lubricating oil to vitiate it.

In the drawings and in the discussion the process of the invention has involved the operation of a pump of the rotary type. However, it should be understood that the efliciencies of the invention apply to some degree to the operation of a reciprocating pump and it should be understood that the scope of the following claims are intended to be of such breadth as they would encompass the operation of a reciprocating pump.

The present invention may thus be embodied in other specific forms without departing from the spirit or central attributes thereof and, accordingly, reference should be made to the appended claims rather than the foregoing specification as indicating the scope of the invention.

We claim:

1. A process for operating a mechanical vacuum pump at temperatures above 200 F. which includes lubricating the pump with a liquid lubricant comprising a silicone and a thickening amount of a copolymer having an intrinsic viscosity in chloroform below about 0.5 and consisting of a doubly bridged chromium atom coordinated with a hydroxyl group and a water molecule, characterized by having at .least two difierent. bridging groups wherein each bridging group is the anion of an acid R R P(0)OH, where R and R are selected from the group consisting of alkyl and aryl containing from 1 to 18 carbon atoms and with the proviso that at least one bridging group contains at least one alkyl group, and said copolymer being terminated at its ends by the anion of an aliphatic carboxylic acid containing from one to four carbon atoms.

2. A process for operating a mechanical vacuum pump at temperatures above 200 F. which includes lubricating the pump with a lubricant comprising an alkyl-aryl silicone and from about 0.1% to about 3% by weight of the composition of a copolymer having an intrinsic viscosity in chloroform below about 0.5 and consisting of a doubly bridged chromium atom coordinated with a hydroxyl group and a water molecule, characterized by having at least two diflferent bridging groups wherein each bridging group is the anion of an acid R R (O)OH where R and R are selected from the group consisting of alkyl and aryl containing from 1 to 18 carbon atoms and with the proviso that at least one bridging group contains at least one alkyl group, and said copolymer being terminated at its ends by the acetate anion.

3. A process as in claim 2 wherein the silicone is methylphenyl silicone and the copolymer bridging groups are derived from methylphenylphosphinic acid and dioctylphosphinic acid.

4. A process as in claim 2 wherein the silicone is methylphenyl silicone and the copolymer bridging groups are derived from dibutylphosphinic acid and dioctylphosphinio acid.

5. A process as in claim 2 wherein the silicone is chlorophenylmethyl silicone and the copolymer bridging groups are derived from dioctylphosphinic acid and diphenylphosphinic acid.

6. A process as in claim 2 wherein the silicone is methylphenylsilicone.

7. A process as in claim 2 wherein the silicone is methylphenylsilicone and the copolymer bridging groups are derived from methylphenylphosphinic acid and dioctylephosphinic acid.

8. A process as in claim 2 wherein the silicone is chlorophenylmethyl silicone.

9. A process as in claim 2 wherein the silicone is methylphenylsilicone and the copolymer bridging groups are derived from methylphenylphosphinic acid and diperfluoroheptyl phosphinic acid.

10. An apparatus comprising a mechanical vacuum pump having a sealing lubricant comprising a silicone and thickening amount of copolymer having an intrinsic viscosity in chloroform below about 0.5 and consisting of a doubly bridged chromium atom coordinated with a hydroxyl group and a water molecule, characterized by having at least two dilferent bridging groups wherein each bridging group is the anion of an acid R R P(O)OH, where R and R are selected from the group consisting of alkyl and aryl containing from 1 to 18 carbon atoms and with the proviso that at least one bridging group contains at least one alkyl group, and said copolymer being terminated at its ends by the anion of an aliphatic carboxylic acid containing from one to four carbon atoms.

11. A vacuum pump for operation at temperatures above 212 F. devoid of any expedient to avoid condensation or revaporization of vapors generated within the pump comprising a housing having an inlet port and an outlet port in communication with a pumping chamber, a pressure responsive valve controlling flow from said chamber through said outlet port, a movable pumping element in said chamber, means of selectively moving said element to produce a suction side in said chamber adjacent to the point where said inlet port is in communication with said chamber, a compression side in said chamber adjacent the point where said outlet port communicates with said chamber, and employing a lubricant which maintains a seal between said movable pumping element of said pumping chamber with the pump chamber wall, wherein said lubricant is comprised of a silicone and a thickening amount of copolymer having an intrinsic viscosity in chloroform below about 0.5 and consisting of a doubly bridged chromium atom coordinated with a hydroxyl group and a water molecule, characterized by having at least two different bridging groups wherein each bridging group is the anion of an acid R R P(O)OH, where R and R are selected from the group consisting of alkyl and aryl containing from 1 to 18 carbon atoms and with the proviso that at least one bridging group contains at least one alkyl group, and said copolymer being terminated at its ends by the anion of an aliphatic carboxylic acid containing from one to four carbon atoms.

References Cited UNITED STATES PATENTS 1,890,574 12/1932 Dubrovin 230-147 XR 3,156,410 11/1964 Le Riche 230--147 XR 3,331,775 7/1967 Saraceno 25232.5

10 ROBERT M. WALKER, Primary Examiner.

US. Cl. X.R. 

